The act of connecting the tip of ones penis to the nipple of another, simply for the sheer fun of it - often proceeded by screaming loudly 'NIPTIPPP!'

Jay: How did it go with that guy last night?
Ida: Well, pretty well until he niptip'd me.
Jay: No fucking way! He niptip'd you?!
Ida: Yep, simple whapped it out, hit my nip with the tip and screamed 'THISS ISSS NIPTIPPP!'